This invention relates generally to semiconductor resistive networks and, more particularly, to a semiconductive series resistive network comprised of resistors which may be individually trimmed.
The above referenced patent application discloses a diffused resistor on an integrated circuit die which may be trimmed by metal migration. This is accomplished by pulsing the resistor with high amplitude, small duty cycle current pulses. The approach is especially useful in production and has definite advantages over laser trimming, link blowing, or zener zapping. First, the resistors can now be trimmed after the integrated circuit is packaged. Second, the resistors trimmed by metal migration (RTMM) occupy very little die area. Third, in some integrated circuits, 25% of the die area is required for trim and protection circuitry. Using RTMM resistors, a test computer performs a trim algorithm, not digital logic on the chip. Fourth, RTMM resistors exhibit increased accuracy and resolution. That is, they undergo resistance changes as small as 25 milli ohms/pulse on a 25 ohm resistor. Finally, no additional expensive equipment is required. The part is trimmed at the same time it is probed or finally tested using the same equipment.
Assuming a silicon substrate and aluminum contacts, when small duty cycle current pulses are applied to the RTMM resistors, electron momentum exchange causes some movement of silicon and aluminum atoms in the direction of current flow, commonly referred to as electromigration. As the resistor temperature rises, however, the ability of aluminum to dissolve silicon increases. This phenomenon causes an aluminum filament to grow from the positive terminal, dissolving silicon as it grows, toward the negative terminal, against the flow of electrons. As the filament grows, the resistive value decreases.
It should thus be clear that no more than two trim pads are necessary to trim an RTMM resistor. Obviously, however, many circuits include more than one resistor which must be trimmed and it would be impractical to provide two trim pads for each such resistor.